1. Field of the Invention
The present invention relates to a control system for a fuel cell. More specifically, the present invention relates to a control system for a fuel cell which controls a pressure-flow characteristic of a reactant gas which, for instance, is supplied to a fuel cell provided with a reforming device.
2. Description of Related Art
A conventional solid polymer membrane type fuel cell includes a cell in which a solid polymer electrolyte membrane is sandwiched by an anode and a cathode, and a stack is formed by a plurality of laminated cells (hereinafter referred to as a fuel cell). In general, hydrogen is supplied to the anode as a fuel and an air is supplied to the cathode as an oxidizing agent so that hydrogen ions generated at the anode by a catalytic reaction pass through the solid polymer electrolyte membrane and move to the cathode to electrochemically react with oxygen to generate a power.
A fuel cell unit provided with a reforming device is disclosed, which utilizes an alcohol type compound, such as methanol, or a hydrocarbon type compound, such as gasoline, as a raw material and produces a hydrogen-enriched fuel by modifying such raw materials, for instance, in the Japanese Unexamined Patent Application, First Publication No. 11-329472.
In such a fuel cell unit, a pressure-flow control valve for controlling the flow of a fuel discharged from the fuel cell is provided so that a predetermined generation efficiency of the fuel cell may be ensured by setting a predetermined pressure of the anode side of the fuel cell with respect to the cathode side and that a predetermined output is obtained by controlling the flow of the fuel supplied to the fuel cell.
However, in the above-mentioned example of the conventional fuel cell unit, there are problems that, for instance, if an analog control of the pressure-flow control valve is carried out, it is difficult to control the valve in a stable manner and a response thereof cannot be enhanced, although the opening degree of the valve may be continuously varied.
Also, in a case where only one pressure-flow control valve is used, if a digital control is carried out by using, for instance, a stepping motor, it becomes difficult to accurately control the pressure-flow characteristic of the fuel over an entire flow range, from the low output to the high output of the fuel cell, because the opening degree of the valve per one step of the motor is fixed.
The present invention was achieved in consideration of the above problems and its objectives include providing a control system for a fuel cell which is capable of accurately controlling the pressure-flow characteristic of a reactant gas or discharged reactant gas over a wide output range of the fuel cell.
The present invention provides a control system for a fuel cell, including a reactant gas supplying unit (for instance, the fuel supplying unit 12 or the oxidizing agent supplying unit 17 which will be described later in embodiments of the present invention) which supplies a reactant gas (for instance, a fuel or an oxidizing agent in the following embodiments) to the fuel cell (for instance, the fuel cell 11 in the following embodiments), and a discharged reactant gas flow control unit disposed at an outlet portion of the fuel cell for discharging a discharged reactant gas (for instance, a discharged fuel or a discharged oxidizing agent in the following embodiments) from the fuel cell. The discharged reactant gas flow control unit includes a first control valve (for instance, the large flow valve 28 in the following embodiments); and a second control valve (for instance, the small flow valve 27 in the following embodiments) disposed in parallel with the first control valve, the second control valve having a different pressure-flow control characteristic as compared with the first control valve, wherein only the second control valve controls the flow rate of a discharged reactant gas until the flow rate of the discharged reactant gas exceeds a predetermined value, and the second control valve together with the first control valve controls the flow rate of the discharged reactant gas when the flow rate of the discharged reactant gas exceeds the predetermined value.
In accordance with another aspect of the invention, the first control valve and the second control valve are under digital control so that the opening degree of the first and second control valves may vary in a stepwise manner.
In yet another aspect of the invention, the opening degree of the first control valve per one step is less than or equal to the maximum opening degree of the second control valve.
In yet another aspect of the invention, the first control valve is under feedforward control based on a target value of a pressure-flow characteristic (for instance, the target working pressure, and the flow rate Q1 of a discharge gas in the following embodiment) of a reactant gas or the discharged reactant gas.
In yet another aspect of the invention, the second control valve is under feedback control based on a detected value of pressure-flow characteristic (for instance, the pressure P1, the pressure P2, and the flow rate Q, or the pressure PA, the flow rate QA, the generated current I, and the temperature TA) of the reactant gas or the discharged reactant gas.
According to the control system for a fuel cell having the above-mentioned configuration, in the low flow rate region of the discharged reactant gas, for instance, only the second control valve having a relatively small flow control capacity is used to control the flow and, in the high flow rate of the discharged reactant gas, the first control valve having a relatively large flow control capacity is used in addition to the second control valve to carry out a pressure-flow control of the discharged reactant gas over the entire output range of the fuel cell in an accurate and quick manner.
Also, when a stepping motor having a predetermined number of steps is used as a driving source for the first and second control valves, the first control valve whose opening degree per one step is large is feedforward controlled and the second control valve whose opening degree per one step is small is feedback controlled.
In the low flow rate region of the discharged reactant gas, the first control valve is totally closed and only the second control valve is used to control the pressure-flow characteristic of the reactant gas so that the accuracy to a response may be enhanced.
On the other hand, in the high flow rate region of the discharged reactant gas, the first control valve is subjected to a feedforward control so that the pressure-flow characteristic may be largely varied and, in addition to that, the second control valve is subjected to a feedback control to make adjustment to the changed made by the first control valve. In this manner, it becomes possible to prevent a decrease in accuracy to a response in the high flow rate region of the discharged reactant gas.
In yet another aspect of the invention, the opening degree of the first control valve, which corresponds to the target value of a pressure-flow characteristic of the discharged reactant gas, is determined based on information containing a pressure-flow control characteristic of the first control valve at a predetermined opening degree of the second control valve.
In yet another aspect of the invention, the number of steps for the opening degree of the first control valve is an integer closest to a predetermined approximate value of the target value of a pressure-flow characteristic of the discharged reactant gas, and the second control valve corrects the difference between the opening degree of the first control valve and the predetermined approximate value by a feedback control.
According to the control system for a fuel cell having the above-mentioned configuration, in the high flow region of the discharged reactant gas, for instance, the opening degree of the second control valve may be set at about a half of its maximum opening degree so that it becomes possible to correct in a quick manner the control of the first control valve whose opening degree per one step is relatively large.
In the above mentioned case, there may be a difference between the opening degree of the first control valve whose opening degree per one step is relatively large and the opening degree determined by, for instance, searching through a map based on the target value of the pressure-flow characteristic of the discharged reactant gas. However, the difference may be corrected by the second control valve whose opening degree per step is relatively small so that a control may be carried out with high precision while maintaining an excellent response time.
In yet another aspect of the invention, the reactant gas supplying unit pressurizes air to supply it as the reactant gas to a cathode side of the fuel cell.
According to the control system for a fuel cell having the above-mentioned configuration, it becomes possible to accurately control the pressure-flow characteristic of the air supplied to the cathode side of the fuel cell over the wide flow range from the low output side to the high output side of the fuel cell.
In yet another aspect of the invention, the reactant gas supplying unit reforms a fuel to produce a hydrogen-enriched reformed fuel to supply it as the reactant gas to an anode side of the fuel cell.
According to the control system for a fuel cell having the above-mentioned configuration, it becomes possible to accurately control the pressure-flow characteristic of the reformed fuel supplied to the anode side of the fuel cell over the wide flow range from the low output side to the high output side of the fuel cell.
The present invention also provides a control system for a fuel cell, including a discharged reactant gas flow control unit disposed at an outlet portion of the fuel cell. The discharged reactant gas flow control unit includes: a first control valve which is under feedforward control based on a target value of a pressure-flow characteristic of a reactant gas or a discharged reactant gas, a second control valve disposed in parallel with the first control valve, the second control valve being under feedback control based on a detected value of a pressure-flow characteristic of the reactant gas or the discharged reactant gas and having a different pressure-flow control characteristic as compared with the first control valve, wherein the first control valve and the second control valve are under digital control so that the opening degree of the first and second control valves may vary in a stepwise manner, the opening degree of the first control valve per one step being less than or equal to the maximum opening degree of the second control valve. Only the second control valve controls the flow rate of a discharged reactant gas until the flow rate exceeds a predetermined value, and the second control valve together with the first control valve controls the flow rate of the discharged reactant gas when the flow rate of the discharged reactant gas exceeds the predetermined value.
In yet another aspect of the invention, in the above-mentioned control system for a fuel cell, the opening degree of the first control valve, which corresponds to the target value of a pressure-flow characteristic of the discharged reactant gas, is determined based on information containing a pressure-flow control characteristic of the first control valve at a predetermined opening degree of the second control valve, the number of steps for the opening degree of the first control valve is an integer closest to a predetermined approximate value of the target value of a pressure-flow characteristic of the discharged reactant gas, and the second control valve corrects the difference between the opening degree of the first control valve and the predetermined approximate value by a feedback control.
The present invention also provides a control system for a fuel cell (for instance, a fuel cell 11 in the following embodiment), including: a fuel reforming unit (for instance, a reforming unit 15 in the following embodiment) which reforms a fuel (for instance, a liquid fuel) so as to produce a hydrogen-enriched reformed fuel, a fuel supply unit (for instance, a fuel supply unit 12 and the reforming unit 15 in the following embodiment) which supplies the hydrogen-enriched reformed fuel from the fuel reforming unit to the fuel cell, and a discharged fuel flow control unit (for instance, a discharged fuel flow control unit 19 in the following embodiment) disposed at an outlet portion of the fuel cell. The discharged fuel flow control unit includes a first control valve (for instance, a large flow valve 28 in the following embodiment) which is under feedforward control based on a target value of a pressure-flow characteristic of the reformed fuel or a discharged fuel (for instance, a target anode working pressure, a flow rate Q1 of a discharge gas in the following embodiment), a second control valve (for instance, a small flow valve 27 in the following embodiment) disposed in parallel with the first control valve, the second control valve being under feedback control based on a detected value of pressure-flow characteristic of the reformed fuel or the discharged fuel (for instance, a pressure P1, a pressure P2, and a flow rate Q in the following embodiment) and having a different pressure-flow control characteristic as compared with first control valve, wherein the first control valve and the second control valve are under digital control so that the opening degree of the first and second control valves may vary in a stepwise manner, the opening degree of the first control valve per one step being less than or equal to the maximum opening degree of the second control valve, only the second control valve controls the flow rate of a discharged fuel until the flow rate exceeds a predetermined value, and the second control valve together with the first control valve controls the flow rate of the discharged fuel when the flow rate of the discharged fuel exceeds the predetermined value.
According to the control system for a fuel cell having the above-mentioned configuration, in the low flow rate region of the discharged fuel, for instance, only the second control valve having a relatively small flow control capacity is used to control the flow and, in the high flow rate of the discharged fuel, the first control valve having a relatively large flow control capacity is used in addition to the second control valve to carry out a pressure-flow control of the discharged fuel over the entire output range of the fuel cell in an accurate and quick manner.
Also, when a stepping motor having a predetermined number of steps is used as a driving source for the first and second control valves, the first control valve whose opening degree per one step is large is feedforward controlled and the second control valve whose opening degree per one step is small is feedback controlled.
In the low flow rate region of the discharged fuel, the first control valve is totally closed and only the second control valve is used to control the pressure-flow characteristic of the fuel so that the accuracy to a response may be enhanced.
On the other hand, in the high flow rate region of the discharged fuel, the first control valve is subjected to a feedforward control so that the pressure-flow characteristic may be largely varied and, in addition to that, the second control valve is subjected to a feedback control to make adjustment to the change made by the first control valve. In this manner, it becomes possible to prevent a decrease in accuracy to a response in the high flow rate region of the discharged fuel.
In accordance with another aspect of the invention, the first control valve and the second control valve are under digital control so that the opening degree of the first and second control valves may vary in a stepwise manner, the opening degree of the first control valve per one step is less than or equal to the maximum opening degree of the second control valve, and the first control valve is under feedforward control based on a target value of a pressure-flow characteristic of the reformed fuel or the discharged fuel; and the second control valve is under feedback control based on a detected value of a pressure-flow characteristic of the reformed fuel or the discharged fuel.
In yet another aspect of the invention, the opening degree of the first control valve (for instance, an opening degree SPLBS of the large flow valve 28 in the following embodiment), which corresponds to the target value of a pressure-flow characteristic of the discharged fuel, is determined based on information (for instance, MAP 1 in the following embodiment) containing a pressure-flow control characteristic of the first control valve at a predetermined opening degree of the second control valve (for instance, a half of the maximum opening degree of the small flow valve 27 in the following embodiment), the number of steps for the opening degree of the first control valve is an integer closest to a predetermined approximate value of the target value of a pressure-flow characteristic of the discharged fuel, and the second control valve corrects the difference (for instance, a decimal number in the following embodiment) between the opening degree of the first control valve and the predetermined approximate value by a feedback control.
According to the control system for a fuel cell having the above-mentioned configuration, in the high flow region of the discharged fuel, for instance, the opening degree of the second control valve may be set at about a half of its maximum opening degree so that it becomes possible to correct in a quick manner the control of the first control valve whose opening degree per one step is relatively large.
In the above mentioned case, there may be a difference between the opening degree of the first control valve whose opening degree per one step is relatively large and the opening degree determined by, for instance, searching through a map based on the target value of the pressure-flow characteristic of the discharged fuel. However, the difference may be corrected by the second control valve whose opening degree per step is relatively small so that a control may be carried out with high precision while maintaining an excellent response time.
The present invention also provides a control system for a fuel cell, including: an air pressurizing unit which pressurizes air to produce pressurized air; a pressurized air supply unit which supplies the pressurized air from the air pressurizing unit to the fuel cell; and a discharged air flow control unit disposed at an outlet portion of the fuel cell, the discharged air flow control unit including: a first control valve; and a second control valve disposed in parallel with the first control valve, the second control valve having a different pressure-flow control characteristic as compared with the first control valve, wherein only the second control valve controls the flow rate of a discharged air until the flow rate exceeds a predetermined value, and the second control valve together with the first control valve controls the flow rate of the discharged air when the flow rate of the discharged air exceeds the predetermined value.
In yet another aspect of the invention, in the above control system for a fuel cell, the first control valve and the second control valve are under digital control so that the opening degree of the first and second control valves may vary in a stepwise manner, the opening degree of the first control valve per one step is less than or equal to the maximum opening degree of the second control valve, and the first control valve is under feedforward control based on a target value of a pressure-flow characteristic of the pressurized air or the discharged air; and the second control valve is under feedback control based on a detected value of a pressure-flow characteristic of the pressurized air or the discharged air.
In yet another aspect of the invention, in the above control system for a fuel cell, the opening degree of the first control valve, which corresponds to the target value of a pressure-flow characteristic of the discharged air, is determined based on information containing a pressure-flow control characteristic of the first control valve at a predetermined opening degree of the second control valve, the number of steps for the opening degree of the first control valve is an integer closest to a predetermined approximate value of the target value of a pressure-flow characteristic of the discharged air, and the second control valve corrects the difference between the opening degree of the first control valve and the predetermined approximate value by a feedback control.
The present invention also provides a control system for a fuel cell including: a fuel reforming unit which reforms a fuel so as to produce a hydrogen-enriched reformed fuel; a fuel supply unit which supplies the hydrogen-enriched reformed fuel from the fuel reforming unit to an anode side of the fuel cell; an air pressurizing unit which pressurizes air to produce pressurized air; a pressurized air supply unit which supplies the pressurized air from the air pressurizing unit to a cathode side of the fuel cell; and a plurality of discharged fuel flow control unit disposed at outlet portions of the fuel cell, the discharged fuel flow control unit including: a first control valve; and a second control valve disposed in parallel with the first control valve, the second control valve having a different pressure-flow control characteristic as compared with the first control valve, wherein only the second control valve controls the flow rate of a discharged fuel or a discharged air until the flow rate exceeds a predetermined value, and the second control valve together with the first control valve controls the flow rate of the discharged fuel or the discharged air when the flow rate of the discharged fuel or the discharged air exceeds the predetermined value.
In yet another aspect of the present invention, in the above control system for a fuel cell, the first control valve and the second control valve are under digital control so that the opening degree of the first and second control valves may vary in a stepwise manner, the opening degree of the first control valve per one step is less than or equal to the maximum opening degree of the second control valve, and the first control valve is under feedforward control based on a target value of a pressure-flow characteristic of one of the reformed fuel, pressurized air, discharged fuel, and discharged air; and the second control valve is under feedback control based on a detected value of pressure-flow characteristic of one of the reformed fuel, pressurized air, discharged fuel, and discharged air.
In yet another aspect of the present invention, in the above control system for a fuel cell, the opening degree of the first control valve, which corresponds to the target value of a pressure-flow characteristic of one of the reformed fuel, pressurized air, discharged fuel, and discharged air, is determined based on information containing a pressure-flow control characteristic of the first control valve at a predetermined opening degree of the second control valve, the number of steps for the opening degree of the first control valve is an integer closest to a predetermined approximate value of the target value of a pressure-flow characteristic of one of the reformed fuel, pressurized air, discharged fuel, and discharged air, and the second control valve corrects the difference between the opening degree of the first control valve and the predetermined approximate value by a feedback control.